1. Field of the Invention
The present invention relates to a piezoelectric ceramic and to a piezoelectric element employing it.
2. Related Background Art
Known piezoelectric ceramics exhibit piezoelectricity, whereby application of an electric field generates mechanical strain and stress. Such piezoelectric ceramics are used in various types of piezoelectric elements including actuators, piezoelectric buzzers, sounding bodies, sensors and the like.
Actuators employing piezoelectric ceramics characteristically realize high precision to be obtained for fine displacement and produce large generated stress, and they are used for positioning of precision machine tools and optical devices. The piezoelectric ceramics used in actuators are, in most cases, made of lead zirconate titanate (PZT) which has excellent piezoelectric properties. However, because lead zirconate titanate contains a large amount of lead, concerns have been raised in recent years regarding the effects on the environment by elution of lead caused by acid rain. A demand therefore exists for a piezoelectric ceramic material with a satisfactorily reduced lead content, as a substitute for lead zirconate titanate. Various types of lead-free piezoelectric ceramic materials have been proposed to answer this demand.
Barium titanate (BaTiO3), for example, is known as a lead-free piezoelectric ceramic material. Piezoelectric ceramic materials comprising barium titanate (BaTiO3) and other components in solid solution therewith have also been proposed as a means of improving the piezoelectric characteristics of barium titanate. For example, 3-component solid solutions such as BaTiO3—KNbO3—NaNbO3 have been proposed (Japanese Unexamined Patent Publication No. 2003-252681).
It is widely known that solid solutions for piezoelectric ceramics having multiple components in solid solution exhibit high piezoelectric characteristics at the phase boundary (MPB) of crystal structures that exhibit piezoelectricity, such as at the phase boundary between tetragonal and orthorhombic crystals. The compositions around these crystal structure phase boundaries in 2-component or 3-component solid solutions are therefore being actively researched.
The two-component system phase diagram for BaTiO3—KNbO3 shown in FIG. 3 is disclosed in R. J. Bratton, T. Y Tien, J. Am. Ceram. Soc., 50, 90-93 (1967). Specifically, the crystal structure of a two-component solid solution of BaTiO3—KNbO3 has been recognized as non-piezoelectric cubic crystal in a wide compositional ratio, as shown in FIG. 3. This two-component solid solution has therefore not been considered very seriously as a piezoelectric ceramic material. That is, since in the BaTiO3—KNbO3 two-component system shown in FIG. 3, a crystal structure phase boundary has been recognized in compositions with an extremely high proportion of one component and an extremely low proportion of the other component, it has been assumed that piezoelectricity is exhibited only in a biased composition where the proportion of one of the components is extremely high.